The oscillating movement of the roll stand of a pilger step-by-step rolling mill has the result that the kinetic energy in the roll stand varies greatly over the path. Without suitable countermeasures, the crank drive would attempt to outspeed the motor twice per oscillation period and therefore cause the motor to act as a generator. The periodic change from motor mode operation to generator mode operation arising therefrom results in increased wear of the motor and furthermore increases the energy consumption of the rolling mill. The energy which the motor takes from the crank drive during generator mode operation must be returned to the crank drive during the following motor mode operation.
Thus, it is apparent that, if the motor drive supplies a constant power, equal to the time average power necessary for the operation of the device, that there will be a degree of inefficiency resulting from the time varying course of the power requirements within the operation cycle of the machine. While devices as large flywheels or other mechanical rotation power storage devices do exist which might reduce angular velocity variations, their use poses significant disadvantages due to their size constraints, as well as their adverse effect on startup and shutdown of a mill. In addition, these constant angular momentum devices do not allow the normal variations in crank angular velocity.
However, since the electrical energy produced during generator mode operation cannot be utilized during motor mode operation, this portion of the energy may be seen, to the operator of the rolling mill, as an apparent power which is wasted and unnecessarily increases operating expenses.
To avoid the periodic change from generator to motor operation, two different types of drive constructions have been used. Thus, German Patent No. DE 10 84 451, expressly incorporated herein by reference, proposes to supplement the crank drive device, producing the movement of the roll stand, with a similar crank drive operated 90.degree. out of phase, i.e. in quadrature relationship. In this way, a continuous exchange of kinetic energy can take place between the two crank drives. This method has the advantage that the crank speeds of rotation and the crank moment, as well as the corresponding motor speed of rotation and motor moment, are approximately constant over an operating cycle, but it has the disadvantage that the expense for the construction of the machine is greatly increased. The required space is also increased, since, in general, deep foundations are necessary for the components. Finally, inertia forces having a higher order, which can only be compensated for with difficulty, result from operation according to this method.
Another method of solving the problem of the biphasic operation includes the use of torque balancing by the interposition of transmissions having non-uniform, i.e. varying drive ratios. In the normal case, if a crank drive which moves a roll stand is accelerated to a given speed of rotation and then disconnected from the drive, the speed of rotation of the crank shaft will thereafter change periodically. If there is no friction present, this will be true for very long periods of time. If a transmission having a nonuniform ratio corresponding to the natural periodic variation of the crank shaft, i.e., for a constant drive speed of rotation the transmission, has exactly the same variation of speed of rotation on the driven side which the crank drive would have in a non-driven condition, is inserted between the drive and the crank shaft, then the crank drive can also be driven with a constant motor speed of rotation and a constant motor moment. In such a case, the intermediate transmission must convert a constant speed of rotation into a variable speed of rotation which corresponds to the free variation of the speed of rotation of the crank drive. Since the variations in speed of rotation of the crank drive are dependent on the specific speed of rotation, the transmission behavior of the intermediate transmission also must be variable.
One intermediate transmission which is actually used for the equalizing of torque is the so-called cardan joint, which can be adapted to the variations in speed of rotation by changing the angle of bend in the joint, as known from German Patent No. DE 20 30 995, expressly incorporated herein by reference. The method of DE 20 30 995 has the disadvantage that the structural expense necessary in order to make it possible to adjust the torque equalization to a given speed of rotation, is very high. In addition, when the system is not operating at steady state, such as when the operating speed of rotation is changed (decreased), the motor will enter into a generator mode of operation, unless the equalization of the torque is automatically effected. Furthermore, the influence of the load on the torque equalization cannot be taken into account. Therefore, a system, such as that proposed in DE 20 30 995, with a predetermined variation in the transmission ratio, will not be suitable under all conditions of operation.